1. Field of the Invention
The present invention relates to a CS (Carrier-Suppressed) optical pulse train generating device for generating an optical pulse signal in accordance with carrier-suppressed RZ (Return to Zero) format optical intensity modulation or optical phase modulation, and to a CS optical pulse train generating method using this device.
2. Description of the Related Art
The capacities and long-distance capabilities of transmission of optical communications networks are increasing. There have been proposed various types of formats of the optical signals used in optical communications systems that structure optical communications networks. Among these, several are being put into practical use. A representative optical signal format that is being put into practical use is the optical intensity modulation format that expresses a binary digital signal by the strength of the optical intensity. There are two main types of optical intensity modulation formats, which are an NRZ (Non Return to Zero) format in which the optical intensity is maintained during a continuous “1” signal, and an RZ format in which the optical intensity becomes zero once during a continuous “1” signal.
An RZ format optical signal is generated by an optical intensity modulator optical-intensity-modulating the individual optical pulses that structure an optical pulse train in the optical pulse train that is lined-up orderly at uniform intervals on the time axis. The optical intensity modulation of the individual optical pulses structuring the optical pulse train is the generating of a binary digital signal by selectively cutting-off or transmitting-through the optical pulses that structure the optical pulse train. In order to generate an RZ format optical signal, the optical pulse train is needed in advance, and a light source that generates the optical pulse train is needed.
As described above, an RZ format optical signal is a binary digital signal obtained by optical-intensity-modulating an optical pulse train that is lined-up orderly at uniform intervals on the time axis. Accordingly, “optical pulse signal” and “optical pulse train” have the following meanings hereinafter. “Optical pulse signal” is used to indicate a row of optical pulses serving as a binary digital signal that is obtained by optical-intensity-modulating an optical pulse train that is lined-up orderly at uniform intervals on the time axis. On the other hand, “optical pulse train” is used to indicate the aggregate of optical pulses that are lined-up without deficiencies, orderly at uniform intervals on the time axis.
The RZ format is a format in which the optical intensity becomes zero once even during a continuous “1” signal. Accordingly, generally, the frequency band of the light that serves as an electric field of light is wide as compared with the NRZ format.
In an RZ format optical pulse signal, the optical pulses expressing the bits that mean “1” always exist individually on the time axis. Accordingly, the optical pulse signal is structured as an aggregation of optical pulses whose full width at half maximum (FWHM) are narrow. On the other hand, an NRZ format optical pulse signal is structured as optical pulses that, when bits meaning “1” appear in continuation, have a continuous wide width during the time that the “1” continues. Therefore, the full width at half maximum of the optical pulses structuring an NRZ format optical pulse signal are, on average, wider than the full width at half maximum of the optical pulses structuring an RZ format optical pulse signal.
Accordingly, the frequency band that an RZ format optical pulse signal occupies (hereinafter also called “frequency spectral band” upon occasion) is wider than the frequency spectral band occupied by an NRZ format optical pulse signal. In the following description, there are cases in which simply the term “spectrum” is used in cases in which there are no need to differentiate between whether it is a spectrum expressed by frequency or a spectrum expressed by wavelength.
When the spectral band is wide, first, due to the group velocity dispersion of the optical fiber that is the signal transmission medium, the effect of waveform distortion, in which the full width at half maximum of the optical pulse on the time axis widens, markedly appears, and the transmission length is thereby limited. Secondly, when taking into consideration the increase in capacity in accordance with wavelength multiplexing systems, in order to suppress crosstalk between channels to which adjacent wavelengths are assigned, the difference in the wavelengths assigned to adjacent channels must be made to be large. In either case, an optical pulse signal of a wide spectral band is not preferable from the standpoint of the efficient utilization of the frequency band by the optical communications network that uses that optical pulse signal.
Thus, methods of narrowing the spectral band of an RZ format optical pulse signal have been proposed. A representative method there among is a method that employs a so-called CS-RZ format that RZ-formats an optical pulse train whose phase serving as an electric field of light is inverted between optical pulses that are adjacent on the time axis. (See, for example, A. Hirano, Y. Miyamoto, S. Kuwahara, M. Tomizawa, and K. Murata, “A Novel Mode-Splitting Detection Scheme in 43-Gb/s CS- and DCS-RZ Signal Transmission”, IEEE J. Lightwave Technology, vol. 20, No. 12, pp. 2029-2034, 2002.) The phase that serves as an electric field of light being inverted between optical pulses that are adjacent on the time axis, is synonymous with the phase difference between adjacent optical pulses being π.
Inverting the phase that serves as an electric field of light between optical pulses that are adjacent on the time axis means that the phase serving as an electric field of light is not continuous, and a phase jumping portion where the phase of the electric field of light suddenly changes by π exists between the adjacent optical pulses. Accordingly, the effect of the interference that arises between adjacent optical pulses becomes the effect of offsetting the amplitudes of one another. On the other hand, when the phase that serves as an electric field of light between optical pulses that are adjacent on the time axis is the same phase, the effect of the interference that arises between these optical pulses becomes an effect in which the amplitudes thereof are added together.
In the CS-RZ format, the spectral band can be reduced by about 25% as compared with the usual RZ format in which the phase that serves as an electric field of light between optical pulses that are adjacent on the time axis is the same phase. (Refer to A. Hirano, Y. Miyamoto, S. Kuwahara, M. Tomizawa, and K. Murata, “A Novel Mode-Splitting Detection Scheme in 43-Gb/s CS- and DCS-RZ Signal Transmission”, IEEE J. Lightwave Technology, vol. 20, No. 12, pp. 2029-2034, 2002.) Therefore, the CS-RZ format has excellent resistance to waveform distortion due to the group velocity dispersion of the optical fiber, and excellent frequency utilization efficiency. Further, in the CS-RZ format, even if the duty ratio of the optical pulse signal is high, waveform distortion due to interference between optical pulses that are adjacent on the time axis is suppressed more than in the usual RZ format. Therefore, the widths, on the time axis, of the optical pulses structuring the optical pulse signal can be made to be wider than in the usual RZ format. As a result, the spectral band of the electric field of light can be reduced. Namely, by employing a CS-RZ format optical pulse signal, an optical communications system having an excellent long-distance transfer characteristic/frequency utilization efficiency can be realized.
Here, the duty ratio of an optical pulse is the ratio of the full width at half maximum of that optical pulse with respect to the interval between optical pulses that are lined-up adjacent on the time axis (the pulse duration per one bit, also called the “time slot”). Accordingly, the duty ratio being high means that the full width at half maximum of the optical pulse is wide with respect to the time slot. Namely, if the time slot is fixed and the full width at half maximum of the optical pulse is widened, or if the full width at half maximum of the optical pulse is fixed and the time slot is narrowed, the duty ratio becomes high.
The following four methods have been conventionally proposed as methods of generating a CS optical pulse train that are needed in order to generate a CS-RZ format optical pulse signal.
The first method is a method of using a Mach-Zehnder interferometer type LiNbO3 optical intensity modulator (see, for example, A. Hirano, Y. Miyamoto, S. Kuwahara, M. Tomizawa, and K. Murata, “A Novel Mode-Splitting Detection Scheme in 43-Gb/s CS- and DCS-RZ Signal Transmission”, IEEE J. Lightwave Technology, vol. 20, No. 12, pp. 2029-2034, 2002.) Hereinafter, the LiNbO3 optical intensity modulator will be referred to upon occasion as the LN optical intensity modulator. This method will be described by using, as an example, CS optical pulse train generation in which the repetition frequency is 40 GHz. First, continuance wave (CW) light, which is produced from a CW light source, is inputted to the LN optical intensity modulator. Then, the DC bias level of the control electric signal supplied to the LN optical intensity modulator (which is a sine wave in most cases) is set to the minimum voltage value of the light transmittance. Further, if the LN optical intensity modulator is modulated by an electric modulation signal, whose repetition frequency is 20 GHz and whose intensity amplitude which is the voltage difference between the maximum and the minimum (the peak-to-peak voltage, also called “Vpp” upon occasion hereinafter) is 2 times the half-wave voltage Vπ, a CS optical pulse train of a repetition frequency of 40 GHz is outputted from the LN optical intensity modulator.
In accordance with the first method, even if the wavelength of the CW light source is changed, the change in the characteristic of the optical pulse is small, and therefore, a high-performance, wavelength-variable CS optical pulse train generating light source can be provided. This is because the wavelength dependence of the optical intensity modulating characteristic of the LN optical intensity modulator is small. Further, the first method also has the advantage that the repetition frequency can be changed easily.
The second method is a method using a two-mode oscillation laser. A two-mode oscillation laser is a laser in which the longitudinal mode of the laser oscillation spectrum is formed from two wavelength components, and ideally, the intensities of these two wavelength components are equal. The light output of a two-mode oscillation laser is a CS optical pulse train, and the time waveform thereof is a sine wave. Further, the repetition frequency of the CS optical pulse train that is outputted from a two-mode oscillation laser coincides with the difference in the optical frequencies of the two oscillation longitudinal modes.
The oscillation light of the two-mode oscillation laser is an optical pulse train of a repetition frequency that is equal to the beat frequency of the two longitudinal mode components. For this reason, the two-mode oscillation laser is called a two-mode beat light source. However, there are cases in which a light source, by which there is obtained output light that is formed from two wavelength components whose wavelength spectra have equal intensities, is called a two-mode beat light source regardless of the structure thereof. Thus, a two-mode beat light source that is realized by a single laser element is called a two-mode oscillation laser. When indicating a general pulse light source including pulse light sources that are structured by combining plural laser elements, including this two-mode oscillation laser, the term two-mode beat light source is used. Namely, two-mode beat light source is a wide concept that includes two-mode oscillation lasers.
As will be described later, among pulse light sources that are structured by combining plural laser elements, there is known a light source of a form in which two semiconductor lasers that oscillate in the longitudinal single mode are phase-synchronously driven, and the two output lights that are outputted from these two semiconductor lasers are combined and outputted. Further, there is known a light source that is structured such that two-mode beat light is obtained by extracting only adjacent two wavelength components among the longitudinal mode components by a wavelength filter, from output light of a mode-locked semiconductor laser having numerous longitudinal mode components.
A two-mode laser oscillation method is known that uses a mode-locked semiconductor laser with which a chirped grating is integrated, and utilizes the dispersion of the chirped grating (refer to, for example, K. Sato, A. Hirano, and N. Shimizu, “Dual mode operation of mode-locked semiconductor lasers for anti-phase pulse generation”, Technical Digest of OFC 2000, paper ThW3-1˜3-3, 2000). For convenience of explanation, here, three longitudinal modes in the vicinity of the Bragg reflection wavelength of the chirped grating are considered. The frequencies of these three longitudinal modes are, from the low frequency side, fm−1, fm, fm+1. By using the dispersion of the chirped grating, the frequency difference (fm−fm−1) between the (m−1)st order and mth order longitudinal modes, and the frequency difference (fm+1−fm) between the mth and (m+1)st order longitudinal modes, are values that differ more greatly the more that frequency pulling-in due to mode-locking operation does not arise. Here, m is an integer.
When mode synchronization is caused by providing modulation equal to (fm+1−fm) to the mode-locked semiconductor laser, frequency pulling-in does not arise at the (m−1)st order mode, and therefore, the laser does not mode-lock-operate. Namely, the laser two-mode-oscillates.
The above two-mode oscillation laser is not limited to a mode-locked semiconductor laser with which a chirped grating is integrated such as that disclosed in the aforementioned document. Further, the above two-mode oscillation laser is not limited to a mode-locked semiconductor laser. The two-mode oscillation laser can be realized by a laser that integrates a sampled grating (refer to L. A. Johansson, Zhaoyang Hu, D. J. Blumenthal, L. A. Coldren, Y. A. Akulova, and G. A. Fish, “40-GHz Dual-Mode-Locked Widely Tunable Sampled-Grating DBR Laser”, IEEE Photon. Technol. Lett., vol. 17, No. 2, pp. 285-287, 2005), or by a self-pulsating distributed feedback semiconductor laser (refer to C. Bobbert, J. Kreissl, L. Molle, F. Raub, M. Rohde, B. Sartorius, A. Umbach, and G. Jacumeit, “Novel Compact 40 GHz PZ-Pulse-Source based on Self-Pulsating PhaseCOMB Lasers”, Technical Digest of OFC 2004, paper WL5, 2004). In this case, the structure of the element that includes the diffraction grating formation region of the laser that integrates a sampled grating or the self-pulsating distributed feedback semiconductor laser is optimized so as to realize a two-mode oscillation laser.
The third method is a method using an optical pulse light source and an optical delay interferometer. This method will be described by using, as an example, a case of generating a CS optical pulse train of a repetition frequency of 40 GHz. First, an optical pulse light source is readied that generates and outputs a usual optical pulse train in which optical phases between optical pulses adjacent at a repetition frequency of 20 GHz are uniform. Next, this optical pulse train is branched in two by using an optical branching device or the like. By using a delay optical system, a time delay of 25 ps is provided to one of the optical pulse trains that were branched in two, and simultaneously, an optical phase difference of π is provided. Thereafter, by multiplexing both optical pulse trains by using an optical combining device, a CS optical pulse train of a repetition frequency of 40 GHz is generated.
An optical-fiber-type element can be used in the optical branching device and the optical combining device, and in the delay optical system. Further, a method that combines a half mirror and a spatial optical system (see H. Murai, M. Kagawa, H. Tsuji, and K. Fujii, “EA Modulator-Based Optical Multiplexing/Demultiplexing Techniques for 160 Gbit/s OTDM Signal Transmission”, IEICE Trans. Electron., vol. E88-C, No. 3, pp. 309-318, 2005) also can be used.
The fourth method is a method of generating a CS optical pulse train by mode-lock-operating a mode-locked DBR (Distributed Bragg Reflector) laser while adjusting the longitudinal mode wavelength of the resonator mode thereof, so that mode-locking operation arises in a longitudinal mode formed from only two wavelength components that have equal intensities (refer to S. Arahira, H. Yaegashi, K. Nakamura, and Y. Ogawa, “Generation of carrier-suppressed broad pulses from model locked DBR laser operating with two carrier wavelengths”, Electronics Letters, 12 Oct. 2006, vol. 42, No. 21, pp. 1298-1300). In accordance with the fourth method, it is possible to generate a CS optical pulse train by using a single element, and the device can be made to be more compact and less expensive. Further, because the pulse duration of the optical pulse structuring the CS optical pulse train can be changed in a wide range, the pulse duration of the optical pulse can be set flexibly in accordance with the communications system that is used or the like.
However, the following problems to be solved exist in the CS optical pulse train generating methods of the above-described first through fourth related art.
In accordance with the first method, because a continuance wave light source is required separately from the LN optical intensity modulator, the device itself becomes large. Further, the amplitude Vpp of the modulation voltage required by the LN optical intensity modulator is 2Vπ, where Vπ is the half-wave voltage of the LN optical intensity modulator. The half-wave voltage Vπ of a general LN optical intensity modulator is 5 V to 10 V, and therefore, the amplitude Vpp of the modulation voltage is 10 V to 20 V. When converting to electric power with the impedance of the LN optical intensity modulator being 50 Ω, this is a large value of 24 dBm to 30 dBm. Accordingly, the first method is a method necessitating a large amount of consumed electric power.
Supposing a case of utilization in a wavelength multiplex system or the like, a large number of CS optical pulse train generating light sources, which corresponds to the wavelength multiplex number, is required. Accordingly, the amount of consumed electric power being large means that an amount of electric power that increases drastically in accordance with the increase in the number of these CS optical pulse train generating light sources, is necessary. Due thereto, the system itself must become large.
In accordance with the second method, there are the advantages that it is possible to generate a CS optical pulse train by using a single element, and the device can be made to be more compact and less expensive. However, in principle, only a sine wave optical pulse train can be obtained, and flexible setting of the pulse width in accordance with the system specifications cannot be carried out. Further, the controllable width of the wavelength is about several nm which is extremely narrow, and the usable range in practical use is limited.
In accordance with the third method, an optical pulse light source, that has a repetition frequency of a magnitude that is half of the repetition frequency of the CS optical pulse train to be generated, is needed. For example, when generating a CS optical pulse train of a repetition frequency of 40 GHz, an optical pulse light source having a repetition frequency of 20 GHz is needed.
Further, with an optical delay interferometer that is needed in optical phase control between optical pulse trains that have been branched in two, there is the need for highly-precise adjustment corresponding to the order of μm with respect to the optical path lengths of the two branched optical pulse trains. Namely, the structure of the device becomes complex, and a highly-precise optical path length controlling circuit is required. As a result, a device for realizing the third method is large and expensive.
In accordance with the fourth method, the wavelength width over which the wavelength of the CS optical pulse train to be generated can be varied is about several nm which is narrow. Supposing a case in which the CS optical pulse train is applied to a large-capacity communications system in accordance with a wavelength multiplexing method, it is desirable that the wavelength variable range of the optical signal source be such that the wavelength can be varied at least about one band of the frequency band, due to requirements such as combining the wavelength into the prescribed wavelength grid of the system, ensuring a spare light source, and the like. For example, because the frequency band width of the C band is 1535 nm to 1565 nm, realization of the ability to vary the wavelength in a width of about the frequency band width of this C band is desired of a CS optical pulse train generating device.